Most coal as it occurs in nature contains some sulfur which is converted into gaseous compounds when the coal is either burned or gasified. If coal is burned with excess air, most of the sulfur is converted to sulfur dioxide (SO2). If coal is gasified by reaction with steam and a limited amount of oxygen, the sulfur is largely converted to hydrogen sulfide (H2S) and carbonyl sulfide (COS). Some coal liquefaction processes also produce hydrogen sulfide as a by-product. In all of these cases a hot, multicomponent gas stream is produced which needs to be desulfurized for the purpose of controlling environmental pollution. Flue gas produced by coal combustion generally is a mixture of nitrogen, carbon dioxide, water vapor, oxygen, and sulfur dioxide with the latter being present in a concentration less than 0.1 vol. %. The product of coal gasification is usually a mixture of hydrogen, carbon monoxide, carbon dioxide, water vapor, nitrogen, hydrogen sulfide, and carbonyl sulfide. Again, the sulfur compounds are present in small concentrations.
Numerous methods have been proposed for removing the aforementioned sulfur compounds from gas streams, and several of the methods are in current use. One widely used method for desulfurizing flue gas involves scrubbing the gas with an aqueous suspension of limestone particles which react with sulfur dioxide to produce calcium sulfite and/or calcium sulfate. A waste product is produced in the form of a wet sludge which is difficult to dewater and to dispose. Consequently, the sludge is impounded and stored ad infinitum. Furthermore, this method imposes an energy penalty since the flue gas is cooled for wet scrubbing and subsequently reheated for stack disposal.
Another method for desulfurizing coal combustion gases involves contacting the products of combustion with limestone particles in such a way that a dry, granular waste by-product is produced which is a mixture of calcium sulfate and unreacted lime. Here too, the material presents a waste disposal problem.
Limestone has also been proposed for removing hydrogen sulfide and carbonyl sulfide from the fuel gas produced by gasifying coal. In one system, which is becoming commercialized, limestone particles are added to a fluidized bed gasifier where they react with the sulfurous gases to form calcium sulfide. The calcium sulfide particles are treated subsequently in another fluidized bed reactor with air to convert the calcium sulfide into calcium sulfate for disposal.
In all of these methods the waste is difficult to reclaim and reuse. Therefore, the methods consume prodigious quantities of limestone and generate tremendous amounts of waste for disposal.
Lime (CaO) which is derived from the decomposition of limestone(CaCO3) is an excellent sorbent for hot gas cleanup. However, in order to employ lime as a regenerable sorbent, it needs to be strengthened to reduce its friability. Structural based modifiers have been used to try to achieve this.
Alumina has been used as a CaO carrier. Snyder et al. (Snyder, R. B. et al. xe2x80x9cSynthetic Sorbents for Removal of Sulfur Dioxide in Fluidized-Bed Coal Combustors,xe2x80x9d ANL/CEN/Fe-77-1, Argonne National Laboratory, Argonne, Ill., June 1977; Snyder, R. B. et al. xe2x80x9cSynthetic SO2 Sorbents for Fluidized-Bed Coal Combustors,xe2x80x9d J. Air Poll. Control Assoc., 27, pp. 975-981, 1977) introduced CaO into porous alumina pellets by refluxing the substrate in a calcium nitrate solution. Via this method up to 15% CaO was impregnated into the carrier. Wolff (Wolff, H. E. P. Regenerative Sulfur Capture in Fluidized Bed Combustion of Coal: A Fixed Bed Sorption Study. Ph.D. Dissertation, Delft University of Technology, Delft, 1991, pp. 1-177) applied a different method to arrive at a similar product. In their work, the alumina and CaO were combined in-sito via a sol-gel technique. They produced a sorbent formulation that contained approximately 6% calcium. Although sorbents fabricated using these two methods produce extremely strong pellets, the preparation methods are expensive and adsorption capacity in terms of weight gain was too low for economical use (Wolff, 1991).
Several zinc-based sorbents have been proposed for desulfurizing hot coal gas. While the materials have a strong affinity for hydrogen sulfide and carbonyl sulfide at high temperature and can be regenerated, they are expensive and decompose at 700xc2x0 C. and above.
An example of a specific process requiring hot-gas desulfurization is integrated coal gasification combined-cycle power generating systems. Though plants that employ the integrated gasification combined-cycle (IGCC) system provide an efficient means of generating electrical power, the power generating systems call for a sorbent capable of removing H2S and COS from coal gas at near gasifier operating temperature which can be 1255xc2x0 K. (1800xc2x0 F.) or more. The gaseous contaminants, mainly H2S, need to be reduced to less than 100 ppm prior to the coal gas entering the gas turbine (Gasper-Galvin et al. Zeolite-Supported Metal Oxide Sorbents for Hot-Gas Desulfurization. Ind. Eng. Chem. Res. 1998, 37 (No. 10), pp. 4157-4166). To maximize the efficiency of an IGCC plant, an adsorbent material capable of removing these contaminants at exit conditions of the gasifier ( greater than 900xc2x0 C.) is preferable. Among various materials which have been proposed for this service, limestone offers several advantages including low cost and widespread availability. Moreover, after limestone is calcined, the resulting CaO in theory can capture 95% or more of the sulfurous species in coal gas when applied within a temperature range of 1070 to 1570xc2x0 K. (1470 to 2370xc2x0 F.) (Westmoreland, P. R. and Harrison, D. P. xe2x80x9cEvaluation of Candidate Solids for High-Temperature Desulfurization of Low-Btu Gases,xe2x80x9d Environmental Science and Technology, 10, pp. 659-661, 1976). However, lime is soft and friable, and the spent sorbent in the form of CaS is not easily regenerated. Therefore, it has been widely regarded as a material to be used once and then discarded. Unfortunately, materials containing CaS cannot be placed directly in a landfill where they will react slowly with moisture and CO2 under ambient conditions to form H2S.
These problems are not insurmountable. The problem of sorbent regeneration may be overcome, for example, by a new process which converts CaS to CaO by alternately oxidizing and reducing the material (Jagtap, S. B. and Wheelock, T. D., xe2x80x9cRegeneration of Sulfided Calcium-Based Sorbents by a Cyclic Process,xe2x80x9d Energy and Fuels, 10, pp. 821-827, 1996; Wheelock, T. D., xe2x80x9cCyclic Processes for Oxidation of Calcium Sulfide, U.S. Pat. No. 5,433,939, Jul. 18, 1995; Wheelock, T. D., xe2x80x9d Cyclic Process for Oxidation of Calcium Sulfide, U.S. Pat. No. 5,653,955, Aug. 5, 1997). The poor physical properties may be overcome by combining lime with a stronger material to create a composite structure which retains the chemical reactivity of lime and the strength of the second material. Previous investigations have employed the following general methods for producing a calcium-based composite: (1) infusion of a strong inert porous substrate with a calcium compound, (2) pelletization of a powder mixture followed by partial sintering, and (3) a sol-gel technique.
Pelletization provides a cheaper means of manufacturing a sorbent. The traditional sorbent preparation method is to combine CaO with a binder in a mixture. A patent by Voss entitled xe2x80x9cLimestone-based sorbent agglomerates for removal of sulfur compounds in hot gases and methods of makingxe2x80x9d, U.S. Pat. 4,316,813, issued Feb. 23, 1982, described a method for preparing an attrition resistant, highly reactive limestone-based sorbent which involves binding limestone particles with a material such as attapulgite clay or Portland cement. Fine particles of limestone and binder are dry-blended, and then water is added to form a paste which is subsequently agglomerated with a pin mixer or pug mill. The agglomerates are subsequently dried and calcined to produce a sorbent for hot sulfurous gases.
The possibility of utilizing Portland cement in a high temperature sorbent for sulfurous gases was suggested by the work of Yoo and Steinberg (Yoo, H. J. and Steinberg, M. xe2x80x9cCalcium Silicate Cement Sorbent for H2S Removal and Improved Gasification Processxe2x80x9d Final Report, DOE/CH/00016-1494, Brookhaven National Laboratory, October 1983). This described a method for preparing a sorbent by agglomerating type III Portland cement by itself. A revolving drum pelletizer was used to prepare spherical agglomerates in the 1 to 3 mm size range by spraying water onto the cement powder. The spherical agglomerates or pellets were cured subsequently in an atmosphere of 100% humidity for 28 days. Although the relatively strong, cured pellets proved capable of adsorbing either sulfur dioxide or hydrogen sulfide from simulated coal gas at 1273xc2x0 K. (1830xc2x0 F.), their adsorption capacity seemed somewhat limited. Consequently, Portland cement seemed to be a good material for use in a composite structure with limestone since it might contribute to both the strength and adsorption capacity of the product.
There is a great need for inexpensive and reusable sorbents which can be employed at higher temperatures, readily regenerated, and handled without breaking down. Presently available sorbents do not meet all of these criteria.
The present invention does not have the drawbacks of the prior art. The sorbents of the present invention have better mechanical properties than the prior art methods/sorbents, are regenerable, and are inexpensive.
An object of the invention is to provide a sorbent which has improved physical characteristics for use in harsh conditions.
Another object of the invention is to provide a sorbent that is regenerable.
Another object of the invention is to provide a sorbent which is durable and attrition resistant.
Yet another object of the invention is to provide a sorbent which is inexpensive.
A further object of the invention is to provide a sorbent for desulfurization of hot gas streams.
An additional object of the invention is to provide a calcium-based sorbent for desulfurization of hot gas streams.
These and other objects, features, and advantages will become apparent after review of the following description and claims of the invention which follow.
The present invention is for a xe2x80x9ccore-in-shellxe2x80x9d sorbent, a pelletized sorbent which combines a reactive core and a porous protective shell. The reactive core plus protective shell creates a strong composite material capable of interacting with and adsorbing, for example, sulfurous gases at high temperature. The sorbent can be used for removal of sulfurous gases, such as H2S, from hot coal gas, from the combustion products of coal-fired boilers, or the like.
The core-in-shell sorbent has a core of reactive, but comparatively physically weak, material and a strong shell. The shell may be reactive, semi-reactive, or inert, but retains the structural integrity of the sorbent during its use. The sorbent is also preferably able to retain its structural integrity during numerous cycles of use and regeneration.
The composite material can be prepared from limestone and a hydraulic cement. This material has considerable promise as a sorbent for H2S at high temperature. By applying the cement as a coating on limestone pellets, a product is produced which combines the high reactivity of lime with the strength of cement. The coating can be made almost entirely of cement or of a mixture of cement and limestone particles. Although the addition of limestone particles to the coating tends to weaken the compressive strength of the final product, it increases the absorption capacity of the material. In addition to the relative concentrations of cement and limestone in the coating, other important parameters are the coating thickness, the type of cement, and the time provided in a pelletizer for strengthening the coating. Good overall results have been achieved with a coating of calcium aluminate refractory cement and limestone particles which was strengthened by prolonged tumbling and heat treatment at 1000xc2x0 C. While pellets with a coating of Portland cement were very strong initially, they were not as durable after heating and reacting with H2S.
The core-in-shell approach of fabricating structurally enhanced lime sorbents for hot gas desulfurization (HGD) was also employed in a lime-alumina system. Pelletization was the preparation method of choice with this system as well. A suitable shell formulation was found by combining two different alumina powders which differed in mean particle size. Initial sorbent screening led to a formulation that initially contained 90% limestone and 10% alumina in the core. Further development produced a pellet with an overall diameter of 4.80 mm, a shell thickness of 0.78 mm, and a fractional shell volume of 69%. This formulation had a compression strength of 16.4 N/mm (3.7 lb/mm) after heat treatment at 1100xc2x0 C. and an adsorption capacity of 50 g/kg when exposed to 1.1% H2S at 880xc2x0 C. for one hour.
The method of producing the sorbents of the present invention involves first pelletizing powdered limestone in a revolving drum and then coating the pellets with a material in the revolving drum which ultimately forms a strong porous shell. A layered structure is produced which combines the adsorptive properties of the lime core with the strength of the porous shell. The method has been demonstrated with both Portland and refractory cements as well as with sinterable powders. Different hydraulic cements were used as the coating materials. Although most hydraulic cements are extremely strong after curing in a moist atmosphere at low temperature, most of this strength is lost when the pellets are fired to temperatures in excess of 1000xc2x0 C. In order to manufacture a good cement based xe2x80x9ccore-in-shellxe2x80x9d sorbent more of the sorbent material had to be incorporated into the shell to avoid cracking upon calcination. Hence, the most successful sorbent formulation containing cement has a highly reactive core and a semi-reactive shell.
The xe2x80x9cidealxe2x80x9d core-in-shell sorbent, however, consists of a reactive core and an inert shell. This will prevent the shell from undergoing drastic structural changes, which may cause the sorbent to prematurely disintegrate. An inert shell has been achieved using alumina rather than cement. An embodiment of the present invention is the calcium-based reactive core and an inert alumina-based shell.
Though the present work has focused on calcium-based sorbents, the core-in-shell concept can be extended to other sorbent materials such as zinc oxide, zinc titanate, manganese oxide, copper oxide, and iron oxide. Other sorbent materials would be readily known to one of ordinary skill in the art for a particular application.